Have sneakers changed the way we run? Why Barefoot Running?
Humans evolved to run long distances without the assistance of footwear
An estimated 30% of runners experience some type of injury each year, often in the feet or lower legs
Anecdotal evidence suggests that individuals who grow up running long distances without supportive footwear sustain fewer injuries Foot-Strike Techniques
Heel Strike Heel lands first, followed by rotation towards the forefoot
Midfoot Strike Entire foot lands simultaneously
Forefoot Strike Ball of the foot lands first, followed by the heel Kinematics of Running
1) Moment of Impact 2) Impact to Flat Foot 3) Flat Foot to Midstance 4) Midstance to Lift-off Leg Muscles Sneakers vs. Five Fingers 1) Moment of Impact
Ankle is dorsiflexed Ankle is plantarflexed Point of impact = rear of shoe Point of impact = ball of foot Arch is not loaded Arch is loaded for support 2) Impact to Flat Foot
Ankle position stays relatively Ankle dorsiflexes constant Arch stretches and begins to flatten Calf muscles and Achilles tendon stretch 3) Flat Foot to Midstance
Ankle dorsiflexes Ankle dorsiflexes Arch begins to flatten and Arch continues to stretch stretch slightly and flatten 4) Midstance to Lift-Off
Ankle plantarflexes Arch recoils Toes flex Momentum
Inelastic collision = momentum is not conserved Heel-strike: vertical momentum is mostly absorbed by the collision force Forefoot-strike: some vertical momentum is converted into angular momentum as the majority of the mass continues to rotate
Linear momentum: p=mv Angular momentum: L = Iω = I x v/r I = moment of inertia, ω = angular momentum Energy
Energy is not conserved
Lift-off velocities are all larger than just before landing - energy is supplied by the leg muscles
Evidence suggests that tendons and ligaments can store some kinetic energy as potential energy
Forefoot-striking may utilize the arch and Achilles tendon for spring loading on landing
Heel-striking does not appear to stretch tendons until midstance Force and Impulse
Meff = “effective mass” = portion of the body stopping abruptly on landing Heel-strike = foot + lower leg = 6.8% body mass Forefoot-strike = forefoot + some rearfoot + some leg = 1.7% body mass T = impact time
vfoot = velocity of foot g = acceleration due to gravity = 9.81 m/s2 Heel-Strike
Impulse = (0.068x64)[1.209+(9.81)(0.05)] = 7.40 N*s Force = J/Δt = 7.40/0.05 = 147.92 N Forefoot-Strike
Impulse = (0.017x64)[0.964+(9.81)(0.05)] = 1.58 N*s Force = J/Δt = 1.58/0.05 = 31.64 N Impact Transient Putting it Together
Heel-strike Forefoot-strike
Effective mass Foot + lower leg = 6.8% Forefoot + some rearfoot total body mass and leg = 1.7% total body mass Vertical Momentum Absorbed by collision Converted into rotational momentum Spring-loading No Yes
Impact force in 5 ms 147.92 N 31.64 N
Total force F=ma (same) F=ma (same)
Impact transient Present, reduced by Absent! cushioned sneakers Training
Forefoot striking feels unnatural to most runners used to highly supportive footwear
Individuals who learn to run without footwear tend to naturally forefoot-strike
Training barefoot (or with “Five Fingers”) and wearing minimalist footwear with less support can encourage forefoot striking even in sneakers
Conclusions
Decreasing the effective mass on landing reduces the immediate force of impact (0.05 s) Supportive sneakers decrease the impact transient when heel-striking but not to the extent of a forefoot strike Loading of the arch tendons may store kinetic energy as potential energy for take-off during a forefoot strike Training barefoot can strengthen the muscles needed for running with a forefoot strike while wearing sneakers
Further questions How much potential energy is actually stored in the arch? How do impact forces correlate with injury? How much linear momentum is converted to angular momentum? Sources
Lieberman, Daniel E. et al. (2010) Foot Strike Patterns and Collision Forces in Habitually Barefoot versus Shod Runners. Nature 463.10: 531-35. Lieberman, Daniel E., Madhusudhan Venkadesan, Adam I. Daoud, and William A. Werbel. "Biomechanics of Foot Strikes and Applications to Running Barefoot or in Minimal Footwear." Running Barefoot. Harvard University. Web. 07 Apr. 2011.